Electrical filter



April 4, 1939. H. J. NICHOLS ELECTRICAL FILTER Filed Aug. 17,

2 Sheets-Sheet 1 [N VENTOR 9 76 4f, A TTOR/VEYS April 1939- H. J.NICHOLS 2,153,202

ELECTRICAL FILTER Filed Aug. 1'7, 1954 2 Sheets-Sheet 2 INVENTOR ATTORNEYS Patented Apr. 4, 1939 UNITED STATES smc'rmcar. mama Harry J.Nichols, Dayton, Ohio, assignor to International Business MachinesCorporation, New York. N. Y a co poration of New York Application August17, 1934, Serial No. 140,285

4 Claims.

This invention relates to electrical filters and particularly toanti-interference filters for telegraph apparatus. p

The invention is described mainly with reference to printing telegraphsignal impulses, but it is generally applicable to the elimination ofinterference in impulse circuits, and in reshaping, and countingimpulses.

In the operation of printing telegraph apparatus over radio systems, itis particularly desirable to eliminate the effects of static and otherforms of interference before they reach the printing telegraph apparatussince not only are erroneous characters printed, but faulty operation ofthe printing apparatus as to shifting, carriage return, piling up oftypebars, etc. is also liable to occur.

It is therefore a general object of the invention to eliminate theundesired effects above mentioned, and thus to minimize the chance ofimproper operation.

A further object is to provide a filter effective to eliminate unwantedinterference and which will at the same time amplify the desiredsignals.

A further object is to provide means for reshaping and groupingimpulses.

Other objects and features will be in part obvious and in parthereinafter pointed out in connection with the following description,the accompanying drawings, and the appended claims.

In the drawings,

Fig. 1 shows in graphic formthe general character of interferencesignals, the signal impulses, and the shaped output impulsesrespectively.

Fig. 2 shows in diagrammatic form one embodiment of the inventionillustrating the basic elements and the principle of the filteringaction.

Fig. 3 shows in diagrammatic form another embodiment of the inventionemploying the electronic filter arrangement of Fig. 2 applied to thegrid circuit of a three element amplifying tube.

Fig. 4 shows a dual arrangement of the circuit shown in Fig. 3 adaptedto transmit bi-directional impulses.

Fig. 5 shows an embodiment of the invention employing agaseous-discharge triode applied to the grid circuit of a three elementamplifying tube.

Fig. 6 shows another embodiment of the invention utilizing agrid-controlled, gaseous-discharge tube of the triode type.

Fig. 7 shows another embodiment of the invention which may be used as atimed signal excluding device to group or count periodic signalimpulses.

Fig. 8 shows a filter arrangement in accordance with the invention,adapted to group or count periodic signal impulses.

Fig. 9 shows a circuit arrangement according to the inventionparticularly adapted to eliminate interference signals and to amplifywanted signal impulses in a printing telegraph transmission system.

Fig. 10 illustrates graphically the currents and voltages in certainbranches of the circuit arrangement shown in Fig. 9.

In the several figures, like characters represent like parts.

Referring now to Fig. 1, line A indicates graphically the generalcharacter of static or other forms of interference signals which are tobe eliminated, and will be referred to as the unwanted or interferencesignals. Such signals are characteristically of varying frequency andamplitude, but their frequency is ordinarily of a higher order than theequivalent frequency of printing telegraph signals. Line B indicatesgenerally the incomingsignal impulses, referred to as the wantedsignals, which are to be passed on in amplified form. Line C indicatesan amplified, regenerated pr squared" impulse as passed on by thefilter-amplifier to the output circuit and will be referred to as theoutgoing impulse.

Referring now to Fig. 2 in detail, the basic arrangement of theinvention comprises a signal receiving input circuit designated bynumeral it, an integrating or filtering capacitor C! across theterminals of which are applied electrical variations representing thereceived signals, impedance Zl representing the equivalent seriesimpedance of the input circuit to Cl, impedance Z2 representing theparallel impedance across 05, a relay device Tl, current limiting meansZ3 controlling the discharge of current through Tl, a steady potentialsource of electrical energy represented by BI, and an output circuit 20.

TI is preferably an electronic relay device such as a gaseous dischargetube or a vacuum tube biased to or below cut-01f, but may be any relaydevice which operates to energize an output circuit upon the applicationof a predetermined threshold potential to its input circuit. Because ofthe low energy requirement for actuation, its quick response, andpossible high amplification factor, a gaseous discharge tube is thepreferred relay device.

The voltage of BI is preferably less than the operating or break-downvoltage of Ti, this voltage difference being referred to for convenienceas the voltage margin.

As previously stated, Zl represents the equivalent series impedance ofthe input circuit to Cl including the fixed impedance of associatedcircuits and apparatus and any variable impedance added for correctiveor control purposes. Likewise, Z2 represents the parallel impedanceacross Cl including any variable additions for control purposes. Zl isalways a material factor in the filtering action, while Z2 may in somecases be a negligible factor. The variable impedance additions may beomitted, especially in established applications, but it is usually foundpreferable to have a variable component of either Zl or Z2, or both, tofacilitate precise adjustment of the filtering action.

The values of Cl and Zl, and Z2 if present, should be suitable to theduration of the wanted signal impulses, since the filtering action ismainly dependent upon the, time constant of the input circuit includingCl. It is to be observed that increasing Zl increases the time ofcharging Cl, while increasing Z2 has the opposite effect, hence Zl andZ2 are complementary in action. Until TI is energized, the outputcircuit is virtually isolated from the input circuit, hence the effectof the output circuit on the filtering action is usually negligible.

Z3 represents in general the means used to limit the duration and/oramplitude of the outgoing impulses, and may comprise a limitingimpedance, or may include a relay or other cut-off to the input circuit.Their effect is to charge Cl in varying degree depending mainly upon'the capacitance of Cl, the impedances Zl, Z2, and the amplitude andduration of the signal waves composing the train. Only the positiveportions of the waves tend to charge CI in a direction to aid Bl andduring intervening negative portions of the waves, Cl is charged inopposition to Bl. Furthermore, the effect of"Cl, Zl and Z2 is to smoothout potential variations in the input circuit, and such smoothing effectis more pronounced in respect to waves of high frequency. Hence so longas a single positive wave does not charge Cl above the voltage margin,Tl remains un-ionized and no signals appear in the output circuit.Signals of brief duration, even though of high amplitude, may beeffectively filtered out by the arrangement shown.

Assume next that a signal impulse as illustrated by line Bl of Fig. 1,either alone or as a mixed signal containing interference signals, isapplied to input circuit I0. In this case Cl is charged mainly in apositive direction, and after an interval its potential is raised abovethe voltage margin and TI is tripped or ionized. It is a characteristicof gaseous-discharge tubes that when ionized the cathode-anode impedanceis greatly lowered, and a comparatively large current may be caused tofiow in the cathode-anode circuit. Such flow of current may beeffectively regulated by the discharge limiting impedance Z3 (assumingthat the output circuit is of comparatively high impedance), and undersuitable conditions Z3 will also cause TI to de-ionize or cut-off afteran interval. By proper selection of circuit constants, the outgoingimpulse may be caused to have a square top, as indicated by line C ofFig. 1. and the outgoing impulse may be of greater amplitude andduration than the incoming signal. Such amplified, squared" impulses areparticularly desirable in the operation of various types of telegraphicinstruments.

Various characteristics and adjustments of the basic arrangement shownare available to control its filtering action, to obtain amplification,and to control the shape and duration of the outgoing impulse. Themanner of manipulating these factors to obtain desired results inparticular applications will be evident to those skilled in the art, andtherefore does not require detailed discussion.

Referring now to Fig. 3, the basic arrangement of Fig. 2 is shown asapplied to the grid circuit of a three-element vacuum amplifier tube T2of conventional type having an anode or plate, cathode, and controlgrid. The input circuit I0 is shown as coupled to the filter arrangementcomprising integrating capacitor Cl and timing resistor R2 by means oftransformer ll, although other coupling means may be employed ifdesired. A grid suppressor resistor R4 is connected in series with thegrid of T2 to limit the grid current on positive swings and to eliminatethe possibility of TI being held ionized by current flowing through thecathode to grid circuit of T2. A potentiometer PI provides a steadypotential for the plate circuit of T2 and for Tl, while potentiometer P2provides a negative bias for the grid of T2.

The operation of the circuit is as follows: Assume that potentiometer PIis adjusted to provide a potential somewhat below the breakdown voltageto TI, and potentiometer P2 is adjusted to bias T2 to the cutoff point.Under the assumed conditions, the plate current of T2 fiowing throughthe" output circuit 20 is negligible. Now assume that a signal impulseof the proper amplitude and duration is applied to the input circuit l0,chargingCl positively in excess of the voltage margin, thus causing TIto become ionized. Current flows from Pl through Cl, Tl, R3 and returnto Pl via P2. Tl being of low impedance when ionized, the potential atthe junction of R3 and R4, and hence at the grid of T2, becomes morepositive and remains so until Cl becomes charged sufficiently to causeTI to cut oil, restoring initial conditions. Plate current fiows throughT2 so long as the grid of T2 is positive relative to the cut-ofi point,causing a current pulse to flow in output circuit 20. Due to theoperation of the ClR2 combination, signals not meeting the requirementsas to amplitude and duration are excluded from the output circuit.Hence, as in the arrangement shown in Fig. 1, unwanted signals do notappear in the output circuit, while wanted signal impulses appear in theoutput circuit in amplified form,and reshaped, if desired. It will beunderstood that a double amplification of the wanted signals may beobtained, the first amplification resulting from the ability of TI whenionized to swing the grid of T2 to a more positive degree than that dueto the input signal impulses alone, while the second amplificationresults from the amplifying properties of T2. The overall amplificationfactor is the product of the amplification factors of TI and T2.

By adjustment of R3 various wave forms for the output signal impulsesmay be obtained.

Referring now to Fig. 4, the arrangement shown is a symmetrical, dualarrangement of the circuit of Fig. 3 adapted to filter and amplify bothpositive'and negative signal impulses, TI and T2 being responsive topositive impulses and T3 and T4 to negative impulses or vice versa. Theoperation of each half of the dual arrangement is similar to the circuitof Fig. 3, and will be manifest from the description thereof. ResistorsR5 and R6 are shunted across output circuit 20 in-order to producesingle positive and negative outgoing impulses.

Referring now to Fig. 5, TI is a grid-controlled, gaseous-discharge tubeof triode type having a cathode, grid, and anode or plate. In this typeof tube, the breakdown voltage is mainly a function of the potential ofthe grid, whilethe tubedrop voltage when ionized is comparatively low,being usually of the order of to 25 volts. The grid is provided with asuitable bias by potentiometer P2, while a suitable plate voltage isprovided by potentiometer PI. Since the grid of this type of tube losescontrol when the tube is ionized and is normally unable to cause thetube to cut oil, separate cut-off means must be provided. The preferredmeans is a cut-off relay L connected in the anode-cathode circuit asshown, although other alternative means may be employed. The filterarrangement comprises inteter PI supplies plate potential to both tubes.

grating capacitor CI and timing resistor R2 whose operation is identicalwith that described in connection with prior figures, and hence does notrequire further description. The function of resistor R4 is to limit thegrid current when tube TI becomes ionized. The grid has'a negligibleeffect on the flow of plate current, hence the amplitude and form of theoutput impulse must be controlled by the output circuit, while theduration is controlled by the cutting off of the plate current by relayL. Since L performs the function of limiting the duration of theoutgoing impulse, it is equivalent in function to R3 in the otherarrangements.

The arrangement shown in Fig. 5 is characterized by the highamplification factor and heavy plate current obtainable.

Referring to Fig. 6, TI is a grid-controlled gaseous-discharge triode,similar to TI of Fig. 5, but preferably of lower current rating. Atriode amplifier tube T2 of vacuum type. has its grid a circuit coupledto the plate circuit of TI by means of transformer I2. Potentiometer P2supplies a steady grid bias to TI and T2, while potentiome- T2 ispreferably biased to cut-off, while TI is biased in such manner that thepotential rise of Ci due to the wanted signal impulses will exceed thevoltage margin between the voltage supplied by PI and the breakdownvoltage of TI. The filter arrangement comprising integrating capacitor CI and timing resistor R2 is preferably connected in the cathode-platecircuit of TI as shown. Limiting resistor R3 is connected in the samecircuit and assists capacitor CI in cutting off the discharge current ofTI. Limiting resistor R3 should be of relatively high value in order tocooperate with Cl and R2 in their cutoff action.

The operation is as follows: The wanted signals raise the potential ofCl above the voltage margin causing TI to ionize; unwanted signals arefiltered out by the CIR2 combination as before. When TI becomes ionizedand conducting, the ionization current flows through the primary oftransformer I2, the secondary of which swings the potential on the gridof T2 causing a current pulse inthe plate circuit of T2, and hence inthe output circuit 20. The flow of current in the cathode-anode circuitof TI charges CI until the back E. M. F.,across CI plus the IR dropacross R3 causes TI to cut oil. It is to be observed that the wave formand duration of the outgoing impulse are under the control of the gridof T2, which in turn is controlled by the secondary voltage oftransformer I2. There is therefore no direct relation between theimpulses in the input and. output circuits, and a wide variety of outputimpulse forms, together with high amplification, may be obtained. Sincethe breakdown voltage of TI is closely controlled by the bias on thegrid of Ti, the voltage margin may be adjusted within close limits.

Referring now to Fig. 7, an impulse filtering circuit is shown which ischaracterized by a definite exclusion period readily cariable betweenwide limits. The circuit comprises an input circuit ID, a timingcondenser CI, a timing resistor RI connected in series therewith asshown, a gaseous discharge tube TI, and discharge limiting resistor R3.A transformer I4 or other coupling device, is connected in series withTI in such manner as to cause an impulse to appear in output circuitwhen TI is passing current. CI is connected across the circuitcontaining TI and both are held at a potential somewhat below thebreakdown voltage of TI by a battery BI or other source of constantpotential.

The operation is as follows: Assume that uni-, form periodic impulses ofmagnitude just adequate to charge CI above the voltage margin areimpressed in aiding manner on the input circuit. Tl will be tripped bythe first impulse and CI will discharge through TI down to the cutoffvoltage of TI, whereupon TI will be de-ioniwd. The discharge current ofCI through TI will produce a brief impulse in the output circuit theform and duration of which is mainly controlled by R3. The discharge ofCI drops the voltage across TI, thereby materially increasing thevoltage margin; hence if succeeding impulses arrive before the normalvoltage margin is restored by the recharging of CI, TI is not tripped,and such impulses are not transmitted to the output circuit. Thecharging rate of Ci is controlled by RI, hence if RI is large, aconsiderable interval may be' made to occur before CI is again chargedsufficiently to restore the normal voltage margin and permit TI to betripped by the signal impulses. By varying RI, the number of impulsesmissed before the tripping of TI and the consequent passing of animpulse to the output circuit may be selected at will over aconsiderable range, which range may be further extended by varying Ci.Since the elements controlling the charging of CI are of stablecharacter, the ratio of passed to excluded signals when establishedremains constant, provided the input signals are substantially uniform,and the circuit shown may be used to group or count periodic impulses orwaves which are too rapid in occurrence for ordinary counting methods.The input signals may, for example, be produced by a photo-electriccounting device actuated by rapidly moving objects, or by otherpreferred means generating uniform rapid impulses.

Referring next to Fig. 8,.T2 is a vacuum tube biased below cutoff bypotentiometer P2 via input circuit I 0. In the plate circuit of T2 is afilter arrangement which comprises a capacitor CI,

lows: Since T2 is'biased below catch, the plate current which normallyflows through T2 is negligible, and practically all the drop through theplate circuit of T2 occurs across the internal anode-cathode circuit ofT2.

therewith. Now assume that a signal of sumcient amplitude to raise thegrid of T2 above cutofl is applied to input circuit I0. The impedance ofT2 is thus lowered, and plate current begins circuit. 20 during thecurrent discharge through Tl, the wave form being dependent in part uponthe electrical constants of the circuits, including transformer l3. Theduration of this discharge may be controlled by adjustment of R3.

In case the signal applied to input circuit I is not of sufiicientamplitude and duration to permit sufficient current to fiow through T2to cause CI to be charged sufiiciently to trip Tl no output impulseoccurs between incoming signal impulses, and the charge on Cl leaks offthrough R2. Hence a filtering action similar to that provided by thearrangement shown in Fig. 1 is obtained.

Another type of action similar to that of the arrangement shown in Fig.7 may be obtained by suitable adjustment of the circuit constants.Assume that a series of periodic impulses are applied to input circuitl0, each impulse being of sumcient magnitude to raise the grid of T2above the cutoff point, but not of sufiicient magnitude to charge CIabove the voltage margin. Under such conditions each impulse causes T2to add an increment of charge to CI, part of which charge leaks oilthrough R2 during the interval between impulses. After a certain numberof such impulses, TI is tripped and an impulse is transmitted to outputcircuit 20. It is thus evident that the arrangement shown in Fig. 8 canbe caused to group or count rapid periodic impulses'in the same manneras the arrangement shown in Fig. 7. Furthermore, the arrangement shownin Fig. 7 may be connected between the original impulse source and thearrangement shown in Fig. 8, thus providing a cascaded counting circuitof high ratio.

Referring now to Fig. 9, the arrangement of Fig. is shown as applied tothe receiving circuit of a single impulse printing telegraph system.Three stages of the receiving circuit are shown, namely the rectifierstage, limiting stage, and filter-amplifier stage, associated withelectron tubes T3, T2, and TI respectively.

' T3 is shown as a triode vacuum tube, but represents any rectifierdevice. Its function is to apply unidirectional voltage variations tothe grid of limiting tube T2, and may be omitted when unidirectionalvoltage variations are otherwise obtained. T3 is preferably suppliedwith a negative bias normally holding the grid at or near the cut-ofi'potential, hence the plate current flowing through load resistorRS isincreased by received signals.

T2 is preferably a vacuum tube biased to provide a certain normal levelof plate current, and has its grid circuit so coupled to rectifier tubeT3 that an increase of plate current in T3 causes the plate current in1'2 to fall below the normal level. As shown, T2 is self biased bycathode Hence very little voltage exists across Cl-R2, and TI in shuntresistor R1. although other well known means may be used. C2 is thecoupling condenser between the plate circuit of T3 and the grid circuitof T2, and R8 is the grid leak resistor for T2.

TI is a triode gas discharge tube similar to that described inconnection with Fig. 5. The input circuit to the grid of TI ispreferably coupled to the plate circuit of T2 by means of transformerII. The input circuit of TI thus comprises the secondary of transformerll, integrating condenser Cl, and resistor RI. For reasons presently tobe described, the natural period of this circuit when connected as shownis prefererably made twice the duration of the wanted signal impulses.The grid TI is held at proper normal bias by potentiometer P2, whilepotentiometer Pl provides a steady source of potential for theanode-cathode circuit of TI. R4 is a grid current limiting resistor,while L is a cut-oil. relay, all as described in connection with Fig. 5.

The operation of the arrangement is as follows: The incoming signals,including interference signals, are applied to rectifier tube T3, andproduce increased plate current through R8. The rise in current in R9 inresponse to incoming signal impulses is shown graphically as ii in thefirst line of Fig. 10. An increase in current through R9 causes a dropin potential at the connection of C2 with the plate circuit of T3, hencethe grid of T2 is made more negative, and the plate current flowingthrough the primary of transformer II is diminished as indicated by i2of Fig. 10. It is to be noted that since i: can only vary between thenormal value and zero, the current variations through the primary of l lare limited toa certain range regardless of the strength of thesignalsrectified by T3, hence the designation of T2 as the limiting tube.

The input circuit to Tl, having inductance and. capacitance, isoscillatory in nature, and the voltage across Cl due to a currentvariation in the primary of II as indicated by is will be of the generalform indicated by e of Fig. 10. It is well established that when anoscillatory circuit is excited by an impulse of the proper duration inrelation to its natural period, the second alternation of the voltagesurge across the capacitance may exceed the first alternation inamplitude, the theoretical limit being twice the amplitude of the firstalternation. The secondary of transformer I I is so connected to theinput circuit that the first alternation produces a charge on CI in adirection opposing the tripping of TI, while the second alternation isin an aiding direction. Referring to Fig. 10, let E represent thevoltage margin, then when e becomes equal to E, TI is ionized ortripped. is represents the outgoing impulse produced by tripping TI, andit will be observed that this impulse occurs after the expiration of thesignal impulse.

Established theory shows that when the ratio of the duration of theexciting impulse to the natural period of an undamped oscillatorycircuit is within the limits of .2 to .8, the second alternation willexceed the first alternation in amplitude. Damping of the circuit due toresistance will narrow the range of this ratio somewhat, and in practiceit can be taken as a basis of design that the natural period of theinput circuit to TI should be approximately twice the duration of thewanted signal impulses. While RI has some effect on the natural periodof the input circuit, the main function of RI is to provide a means ofcontrolling the damping of the input circuit, and in establishedapplications aiuaaoa may be omitted, the correct amount of damping beingobtained by proper design of transformer The arrangement shown in Fig. 9is effective in filtering out interference signals of different timecharacteristics than the wanted signals even if much stronger than thewanted signal impulses since all signals are limited in effect bylimiting tube T2, and signals materially difl'ering in timecharacteristics from the wanted signal impulses will not producesufiicient voltage across CI to trip Tl. Limiting tube T2 also acts asan automatic volume control, and can be adjusted so that signal impulsesweakened by fading or by other causes will be effective to trigger Ti.

The arrangement shown in Fig. 9 is particularly adapted to printingtelegraph systems operated over radio or carrier frequency channels.

It is to be observed that the various embodiments of the inventionpossess in common an integrating capacitor Cl, a timing impedance Zlcontrolling the charging of CI, a gaseous-discharge tube Ti, a dischargelimiting device Z3 controlling the discharge of TI and a steadypotential source. Also that each arrangement is capable of exercising afiltering or selective action on the signals passing therethrough, andis capable of amplifying the incoming signal. The arrangements shownalso provide means for producing outgoing impulses of a wide variety ofshapes and durations at will.

In the various arrangements, the impedance of the input circuit to Clhas an effect on the action oi. that combination, and in some instances,the input circuits may in themselves have suiilcient capacity orimpedance so that a separate timing element is not required. By the wellunderstood principle of equivalent circuits it is clear, however, thatsuch instances are special cases of the basic circuits herein disclosed,in which and Z represent the equivalent capacitance and impedance of theinput circuit, however distributed. In like manner, the function ofdischarge limiting impedance Z3 may in some instances be embodied in theoutput coupling device or circuit, and in such cases 23 represents theequivalent discharge limiting device applied to gaseous discharge tubeTl What is claimed is:

2. An electrical impulse filter arrangement comprising an input circuitincluding a normally unexcited oscillatory circuit, an electrondischarge device having an input and output circuit associatedtherewith, a grid control element included in the input circuit andmeans normally to bias the grid element to render the devicenonconductive, and means to impress the discharge or the oscillatorycircuit upon the input circuit of the device upon excitation of theoscillatory circuit by signal conditions includng means whereby thesecond alternation of the oscillatory circuit discharge is eflectivesolely to overcome the bias of the grid element thereby rendering theelectron discharge device conductive.

3. An electrical impulse filter arrangement comprising an input circuitincluding a normally unexcited oscillatory circuit, an electrondischarge device having an input and output circuit associatedtherewith, said device being characterized by continued operationunaflected by grid potential after starting, means to impress normally agrid bias potential upon the device to render it non-conductive, meansto impress the discharge of the oscillatory circuit upon the inputcircuit of the device upon excitation of the oscillatory circuit bysignal conditions including means whereby the second alternation of theoscillatory circuit discharge is efiective solely to overcome the gridbias potential thereby rendering the electron discharge deviceconductive, and means in the output circuit of the device to restore thedevice upon operation thereof to a non-conductive the last mentionedmeans comprises relay means.

HARRY I. manor-s.

